Electricity storage is an important enabling technology for effective use of renewable energy sources such solar and wind. There are two broad categories of electricity storage applications based on the duration of storage: short durations, from a fraction of second to about one hour, and long durations, from a few hours to ten or hundred of hours. The short duration types are typically used for power support to ensure the reliability and quality of electrical power for which there are technologies in the early stage of commercial application. Long duration types are needed for applications to separate the times between generation and use of electricity at low cost. At present there is a lack of commercially viable technology for long duration type of electricity storage except for pumped hydro. However, pumped hydro is limited by availability of suitable lands due to geological environmental constrains.
Metal-air, particularly zinc-air, electrochemical systems have been seen as promising technologies for low cost large scale energy storage. There have been continuous attempts to develop energy storage systems based on zinc-air chemistry including rechargeable batteries, mechanically and hydraulically rechargeable fuel cells (see review articles by X. G. Zhang: “Zinc Electrodes”, and S. Smedley and X. G. Zhang, “Zinc-Air: Hydraulic Recharge”, in Encyclopedia of Electrochemical Power Sources, Eds. Jungen Garche etc., Amsterdam: Elsevier, 2009).
Electrically rechargeable zinc-air batteries have high energy density. The main technical issues have been fast degradation of the bi-functional air cathode and the detrimental change of the morphology of zinc anode during cyclic discharging and charging. Numerous development efforts have been made to resolve these technical challenges. Some recent developments can be appreciated from the US patent applications, for example, US2010/0021303 and US2010/0316935.
For zinc-air fuel cells, the zinc active anode material is like fuel and can be generated and regenerated by electro deposition. The generation of zinc material by electro deposition serves the function of storing electricity. The deposited metallic material together with electrolyte in fluidic form is fed or fueled into the fuel cells, which serves the function of generating electricity from the stored energy in the metallic zinc. Regenerative zinc fuel cells are ideal for economical long duration energy storage for three fundamental reasons: 1) power generation and energy storage are separated such that energy can be stored independently at low cost; 2) zinc has a high energy density, highest among the common metals that can be reduced in aqueous electrolytes and 3) zinc is inexpensive, one of the lowest cost metals in the market.
Regenerative zinc-air fuel cell systems have many advantages over rechargeable battery systems such as independent scaling of power and capacity and continuous discharging without interruption for charging. Many development efforts have been made on zinc fuel cell technology as can be appreciated in the patent literature, for examples, U.S. Pat. Nos. 5,434,020, 5,849,427, 6,706,433 and US Patent Application US2010/330437. The main technical challenges have been clogging or jamming during fuelling or transporting the zinc materials into and out the electrochemical cells and uneven distribution of the materials within a cell and between the cells. It is essential to have solutions to resolve these technical problems for zinc air fuel cell to function reliably and efficiently.
Metal-redox flow batteries, particularly zinc-redox flow batteries, are another technology system that has been considered having the potential for low cost energy storage. Redox couples of bromine, cerium and iron have been used for development of zinc-redox flow battery technologies, as indicated in literature: Progress in Flow Battery Research and Development (by M. Skyllas-Kazacos et al in Journal of The Electrochemical Society, Vol. 158 (8) R55-R79), 2011, US 2013/0252062 A1, U.S. Pat. No. 8,293,390, and U.S. Pat. No. 5,607,788. As well, iron-redox flow battery has also been explored as disclosed in US 20140065460 A1. However, in the current designs of metal-redox flow batteries the capacity of the batteries is limited by the thickness of the metal anodes. It will be advantageous if the capacity of the metal-redox flow batteries is not limited by the thickness of metal anodes and thus the energy capacity can be scaled independent of power generation.